Standard Loran, or long range, navigational systems consist of a series of radio frequency transmitter stations spaced apart from one another at fixed ground locations. These systems are used in navigation to determine position by measuring the time difference in the arrival of synchronized, radio frequency pulses transmitted from the transmitter stations in a given Loran series or chain.
The most commonly employed Loran navigational system in use at the present time is called Loran-C. Developed during World War II for military use, Loran-C has been used commercially since the mid-1970's, and overcomes many of the shortcomings of earlier long range navigational systems.
Loran-C is a pulsed, low-frequency (100 kilohertz) hyberbolic radio navigational system. Each Loran-C chain consists of three or more synchronized ground stations, each of which transmits radio pulse trains having, at their respective starts of transmission, a fixed time relation to each other. The first station to transmit in a given Loran-C chain is referred to as the master station, while the other stations in the chain are referred to as the secondary stations. The pulse trains are received by receiving equipment that is generally located on board a ship or aircraft whose position is to be accurately determined.
The pulse trains transmitted by each of the master and secondary stations consist of a fixed number of phase coded pulses and are repeated at a constant precise repetition rate. Each pulse in the pulse train has an exact envelope shape and is separated in time from a prior pulse in the same train by a precise fixed time interval (i.e., 1000 microseconds). In addition, the secondary pulse train transmissions are delayed a sufficient amount of time after the master station pulse train transmissions to assure that the secondary pulses are received after the master pulses regardless of the location of the receiving equipment within the coverage area of the particular Loran-C chain. Since the transmissions from the master and secondary stations are in the form of pulses of radio frequency electromagnetic energy which propagate at a constant velocity, the difference in the time of arrival of pulses from a master and a secondary station represents the difference in the length of the transmission paths from these stations to the Loran-C receiving equipment.
The locus of all points on a hydrographic chart corresponding to a constant difference in distance from a master station and a given secondary station, and thus to a fixed time difference of arrival of their respective pulse trains, defines a hyperbola. Loran-C hydrographic charts are available from the National Oceanagraphic and Atmospheric Administration (NOAA) and the U.S. Defense Mapping Agency which are printed with hyperbolic curves marked with time difference of arrival information for the master and each secondary station associated with the particular geographical area covered by the chart. By measuring the time difference of arrival of pulses from the master station and each of two or more secondary stations in a given Loran-C chain, two or more corresponding hyperbolae can be located on such a chart, and their point of intersection accurately identifies the position of the Loran-C receiver. As a result of the use of a moderately low frequency, such as 100 kilohertz, which is characterized by low attenuation, and of extremely precise pulse shaping, timing and synchronization procedures at the transmitter stations, the modern-day Loran-C system is capable of providing equipment position location accuracies within about two hundred feet, and a repeatability within about fifty feet.
Those desiring a more detailed description of the theory and operation of the Loran-C radio navigational system are referred to an article by W. P. Frentz, W. Dean and R. L. Frank entitled "A Precision Multi-Purpose Radion Navigation System", 1957 I.R.E. Convention Record, Part 8, page 79, and to a pamphlet distributed by the Department of Transportation, U.S. Coast Guard, Number CG-462, dated Aug., 1974, entitled "Loran-C User Handbook".
To make the precise time difference of pulse arrival measurements required for the Loran-C positional accuracy, the zero crossing of a specific (usually the third) carrier frequency cycle of each pulse must be identified. These zero crossings are used as tracking points in each pulse to make the time difference of pulse arrival measurements in a well known manner. In theory this will work, but in actual practice, noise and spurious signals at and about the same frequency as the carrier frequency of the Loran-C pulses make the task very difficult. The strength of the received pulses very often is weak and, when weak signals are combined with received noise within the passband of the Loran-C receiver, very low signal-to-noise ratios result. Thus, identification of the desired tracking point of each pulse becomes very difficult and many times impossible utilizing prior receiving equipment. In addition, received noise can distort the envelope of the received pulses and cause erroneous identification of the desired tracking point of the pulse. These problems can in turn result in faulty time difference of pulse arrival measurements, and substantially degrade the accuracy and reliability of the Loran-C equipment positional determinations.
U.S. Pat. No. 4,318,105, which is assigned to the same assignee as the present application, discloses improved Loran-C receiving equipment which provides considerably faster response and measurement times than prior receiving equipment, particularly in extremely low signal-to-noise environments. The receiver disclosed in that patent utilizes a unique digital circuit arrangement and an embedded microprocessor to automatically identify pulses from Loran transmitting stations and make standard hyperbolic navigational measurements.
Briefly, the receiver disclosed in U.S. Pat. No. 4,318,105 operates initially in a so-called "acquisition" mode in which it receives all signals that appear within a small bandwidth centered upon the 100 kilohertz operating frequency of the Loran-C network. In the acquisition mode, the receiver generally locates the pulse trains from the master and secondary stations in a selected Loran-C chain. This is accomplished by coupling the received signals as they are received serially through the time-delaying stages of a multi-stage digital shift register, the outputs of which are in turn coupled to logic circuitry. The shift register and logic circuitry continuously check the received signals to search for the existence of pulse trains having the unique number of pulses, pulse time spacing and phase of those which are transmitted by the master and secondary stations in the selected chain. The embedded microprocessor analyzes the outputs from the shift register and logic circuitry to determine when signals from the master and secondary stations in the selected chain are being regularly and reliably received. Once this determination is made, the microprocessor switches the receiver into a so-called "settling" mode.
In the settling mode, the microprocessor and associated circuitry in the receiver identify the desired tracking point (e.g., the third cycle zero crossing) of each received pulse. In the event that the tracking point of a pulse is not identified at the approximate time indicated by the microprocessor, the analyzation circuitry indicates to the microprocessor whether to add or subtract an increment of time to the approximate time of arrival and then repeats the analyzation procedure. This analyzation procedure and shifting of the approximate search point is repeated automatically until the desired tracking point of the received pulses from each of the master and secondary stations in the selected chain is accurately identified.
Once the desired tracking point is so identified, the microprocessor switches the receiver into a so-called "tracking" mode. In the tracking mode, the receiver continuously tracks the identified tracking points of the received pulses and makes a plurality of time difference of arrival measurements between the pulses from the master and secondary stations of the selected chain using a crystal controlled clock internal to the receiver. The microprocessor then averages these measurements and provides appropriate outputs corresponding to the averages to a visual output display.
U.S. Pat. No. 4,325,067, which is also assigned to the same assignee as the present application, discloses further improvements in microprocessor-controlled Loran-C receiving equipment. The equipment disclosed in the latter patent operates essentially in the same manner as that disclosed in the former patent in terms of its acquisition, settling and tracking functions. In addition, it includes certain additional circuitry which makes it more effective in reducing the effects of noise and spurious signals on the operation and accuracy of the receiver.
More specifically, the receiver disclosed in U.S. Pat. No. 4,325,067 includes circuitry for periodically inverting or shifting the phase of the received signals by 180.degree. at the front end of the receiver immediately after its receiving antenna. These periodic phase reversals cause noise generated internally of the receiver to average out, so that the noise does not introduce a bias level to the received signals which can result in erroneous time difference of signal arrival measurements. The periodic phase reversals are later removed or compensated for downstream of the receiving antenna in the receiver signal path before the time period of the 180.degree. phase shift can interfere with the time difference of signal arrival measurements made by the receiver.
The receiver disclosed in U.S. Pat. No. 4,325,067 also performs various noise checking routines during its settling and tracking modes. In one such routine which is operative during the settling and tracking mode, the receiver, through appropriate time gating with its microprocessor, generates a pair of search windows, with one such window on each side of the expected time of arrival of the tracking point of each pulse being tracked. The envelope slope of the signals in each search window are sampled by the receiver, and the microprocessor develops histograms for a large number of such samples. The histograms are analyzed by the microprocessor to decide if the calculated time of arrival of the tracking point and time position of the search windows should be shifted or revised. The histograms enable the microprocessor to reduce, statistically, the effects of external noise and spurious signals that occur within the search windows. Thus, the desired tracking point of each Loran-C pulse is easier to track, and time difference of arrival measurements are made more accurately, even in noisy signal environments where the signal-to-noise ratios are extremely low.
The receiver disclosed in U.S. Pat. No. 4,325,067 also includes circuitry for avoiding the effects of skywave interference. Skywaves are echoes of the transmitted Loran-C groundwave pulses which are reflected back to earth from the ionosphere. Because of their longer transmission paths, skywaves arrive at a given point after their corresponding groundwaves. The delay may be such that the skywave overlaps either its own corresponding groundwave or the groundwave of a succeeding pulse upon receipt at a Loran-C receiver. In either case, the received skywave can interfere with a groundwave being tracked, and this interference can in turn result in positional errors.
The circuitry disclosed in U.S. Pat. No. 4,325,067 for avoiding skywave interference operates again in the settling and tracking modes of the receiver, and again through appropriate time gating with the microprocessor. The circuitry samples the received signals at several fixed times prior to the expected time of arrival of the tracking point of each pulse being tracked. If the microprocessor determines from these samples that there is an earlier signal having the same repetition rate as the signal being tracked, it modifies its calculated time of arrival and the procedures are repeated. This continues until no signal having the same repetition rate is detected from the sampling, thus confirming that the receiver is properly locked onto the groundwave.
Receivers embodied in accordance with the teachings of the above patents have marked advantages over prior receivers in terms of their ability to acquire, settle and track Loran-C signals accurately and reliably in the presence of both internal and external noise. However, such receivers, like prior receivers, are still susceptible to interference from so-called impulse noise. Impulse noise, as the words imply, consists of fast acting power surges or transients which typically result from sources external to the receiver such as lightning, spark plug ignition or the like. The front end circuitry of a Loran-C receiver is characterized by a relatively high Q factor due to its narrow bandwidth and permanent tuning to the Loran-C operating frequency of 100 kilohertz. This circuitry is thus readily excited by impulses. An impulse may occur in time coincidence with the earlier portion of a Loran-C pulse being tracked by the receiver, but the more probable and troublesome interference situation is one in which the impulse arrives ahead of the pulse being tracked. In the latter situation, the impulse causes front end ringing or energy storage at the Loran-C operating frequency which artifically stretches in time so as to give the appearance of a valid Loran-C pulse. This ringing often results in an erroneous identification of the tracking point, and makes it difficult to track that point due to the effective reduction in signal-to-noise ratio which results from the stored impulse energy. Given the typical time constants of Loran-C receiver front ends, impulses which arrive anywhere up to several hundred microseconds ahead of the tracking point can cause these problems.
Notwithstanding the significant advances that have been made in recent years in Loran-C receiver noise reduction, impulse noise remains as one of the primary, accuracy degrading interference mechanisms in Loran-C systems today. Thus, there exists a real need in the art for improved circuitry and techniques to minimize the effects of impulse noise in Loran-C and related systems.